How Cooling Towers Work: Guide For the Non-Engineer

"Cooling Tower Heat Transfer 101"

By Brad Buecker

[Excerpts]

"Evaporation is utilized to its fullest extent in cooling towers, which are designed to expose the maximum transient water surface to the maximum flow of air – for the longest period of time.”1

For water to evaporate it must consume a large amount of energy to change state from a liquid to a gas.

Figure 1

Figure 1 shows process conditions that could easily exist in a cooling system. We will calculate the mass flow rate of air needed to cool 150,000 gpm of tower inlet water to the desired temperature. We will also calculate the water lost by evaporation (go to link for full calculation). So, with an inlet cooling water flow rate of 150,000 gpm (1,251,000 lb/min), the calculated air flow is 1,248,000 lb/min, which, by chance in this case, is close to the cooling water flow rate. (Obviously, the air flow requirement would change significantly depending upon air temperature, inlet water temperature and flow rate, and other factors, and that is why cooling towers typically have multiple cells, often including fans that have adjustable speed control). The mass balance of water = 146,841 gpm. Thus, the water lost to evaporation is 3,159 gpm. A very interesting aspect of this calculation is that only about 2 percent evaporation is sufficient to provide so much cooling.

Evaporation causes dissolved and suspended solids in the cooling water to increase in concentration. This concentration factor is (logically) termed the cycles of concentration (C). Cycles of concentration can be monitored by comparing the ratio of the concentration of a very soluble ion, such as chloride or magnesium, in the makeup (MU) and recirculating (R) water. Very common is a comparison of the specific conductivity of the two streams, particularly where automatic control is utilized to bleed off recirculating water when it becomes too concentrated.

Besides blowdown, some water also escapes the process as fine moisture droplets in the cooling tower fan exhaust. This water loss is known as drift (D). Where towers are well-designed, drift is quite small and can be as low as 0.0005 percent of the recirculation rate.2 Drift particulate minimization is very important, as regulations on particulate emissions from cooling towers continue to tighten. Leaks in the cooling system are referred to as losses (L).

Reference:

1. J.C. Hensley, ed., Cooling Tower Fundamentals, 2nd Edition; The Marley Cooling Tower Company (now part of SPX Cooling Technologies, Overland Park, Kan.), 1985.
2. Personal conversation with Rich Aull of Brentwood Industries.

Power Engineering, July 2010

AAEA Testifies at Nuclear Power Plant Water Quality Hearing

AAEA President Norris McDonald, right, presented testimony before two administrative law judges at a hearing near the Indian Point nuclear power plant. The hearing was held in Cortlandt, New York at Colonial Terrace. The hearing was to address the New York Department of Environmental Conservation's denial of Entergy's Water Quality Certificate Application.

Excerpts of McDonald's written statement:

"AAEA disagrees with DEC's denial of Entergy's Water Quality Certificate and will provide information that the agency overlooked in its evaluation of the certificate application. Entergy's application, including the addition of a cylindrical wedge-wire (CWW) screen system (as proposed in Entergy’s February 12, 2010, submission), not only complies with existing New York State water quality standards, it also enhances those standards.

But nowhere in its discussion of these “other impacts” is there any acknowledgement by the DEC of the air impacts its decision will have on minority communities. This omission is egregious, particularly in light of the DEC’s numerous policy pronouncements, including DEC Policy Statement CP-29: Environmental Justice and Permitting, issued on March 19, 2003, where DEC expressed its commitment to environmental justice. In Policy Statement CP-29, DEC stated:

'It is the general policy of DEC to promote environmental justice and incorporate measures for achieving environmental justice into its programs, policies, regulations, legislative proposals and activities. This policy is specifically intended to ensure that DEC’s environmental permit process promotes environmental justice.'

CP-29 applies to permit applications received after its effective date (March 19, 2003) and the WQC application was submitted to the DEC on April 6, 2009. Thus, CP-29 clearly applies to this certification process.

To date, DEC’s permitting procedure for the Indian Point 2 and 3 facilities, and in particular, DEC’s Notice of Denial, and other considerations and investigations for these facilities, wholly ignores the issue of environmental justice and turns a blind eye to the significant harm to human health. These substantive and significant deficiencies render the Notice of Denial null and void, and preclude the DEC denying the WQC."

Wedgewire Screens- Best Available Control Technology

Cylindrical Wedgewire Screens would significantly reduce entrainment and impingement of Hudson River fish.

Wedgewire screens allow water to be “filtered” prior to entering the plants' cooling system eliminating the possibility of clogging pumps.

Fish, fish larvae, and fish eggs larger than the slot size are excluded from the intake screens.
Flow-through slot velocity (0.5fps or less) eliminates the possibility of extrusion.

Wedgewire Screens in operation at Unit 2 in April 2013/14, and at Unit 3 in April 2014/15.
Wedgewire screens can be installed in 5 years.

Wedgewire Technology offers the best environmental solution to protect human health, the environment and the fish populations in the Hudson.

The screens can be installed by 2015, and begin to further enhance fish protection efforts a full 15 years ahead of Cooling Towers.

Cooling Towers pose significant environmental and permitting problems that are highly likely to generate numerous law suits that will delay the permitting and construction if in fact they can be permitted.

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